COEVAL 40AR/39AR, U-PB, AND ASTRONOMICAL CLOCKS IN THE EARLY EOCENE

U-Pb ages of zircon from the Analcite and Firehole ash beds in the Eocene Green River Formation are indistinguishable from ⁴⁰Ar/³⁹Ar ages of sanidine in the same beds after adjusting the latter to the astronomically-calibrated age of 28.201 Ma for Fish Canyon sanidine standard. Seven of eight analyses of zircon from the Analcite ash zircon yield a weighted mean ²³⁸U-²⁰⁶Pb age of 49.23 ± 0.18 Ma (MSWD = 6.7; full external 2σ uncertainty) whereas the sanidine ⁴⁰Ar/³⁹Ar age is 49.24 ± 0.18 Ma (full external 2σ uncertainty). Six out of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean ²³⁸U-²⁰⁶Pb age of 51.66 ± 0.25 Ma (MSWD = 11.2), indistinguishable from the recalibrated ⁴⁰Ar/³⁹Ar age of 51.40 ± 0.25 Ma. High MSWD values for U-Pb ages likely reflect some combination of minor inheritance, Pb-loss, and possibly magma chamber residence. Synchronized U-Pb and ⁴⁰Ar/³⁹Ar ages provide and improved means of testing recent time series analyses of the Wilkins Peak Member of the Green River Formation. Calibrating Green River Formation ⁴⁰Ar/³⁹Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of the Green River Formation to the Laskar et al. (2004) astronomical solution for Early Eocene insolation. From this comparison, it appears that the long and short eccentricity cycles exerted a primary influence on deposition of carbonate versus siliciclastic facies.